1. Field of the Invention
The present invention relates to a method of drawing an optical fiber by which an optical fiber is drawn.
2. Related Background Art
In general, an optical fiber is surrounded by a resin coating in order to protect the optical fiber and so forth. Known as an example of such a resin coating is one using a thermosetting resin, such as silicone, as described in Japanese Patent Publication No. SHO 56-49865. When a thermo-curable resin is used, the resin coating cures in a heat-curing furnace, whereby the optical fiber coming out of the heat-curing furnace attains a high temperature. In view of this, there has been an idea of cooling the optical fiber between the heat-curing furnace and a capstan for taking up the fiber. Also, there are optical fibers using a UV-curable resin as the resin coating. In this case, the temperature of the optical fiber coming out of the UV-curing furnace is not so high as in the case of the optical fiber using a thermo-curable resin. Hence, it has not been considered necessary to cool the optical fiber between the UV-curing furnace and the capstan when a UV-curable resin is used.
Japanese Patent Application Laid-Open No. SHO 62-241846 describes a case of tandem coating in which primary coating and secondary coating are carried out by use of UV-curable resins. This publication states the necessity to cool the optical fiber on the upstream side of a secondary die for forming the secondary coating since the application of secondary coating becomes unstable if the temperature of optical fiber rises before the secondary coating is formed. However, there is no consideration concerning the cooling between the secondary die and capstan.
In optical fibers with an outside diameter of 250 xcexcm after being coated with a resin, which are employed in general, the heat capacity of their coated part is so small that temperature does not rise much due to the heat of reaction in the resin-coated part, whereby they can fully be cooled by air cooling. By contrast, optical fibers with a thicker resin coating, which have conventionally been in existence, are drawn slowly to draw and can fully be cooled by air cooling.
An optical fiber is pulled while the periphery thereof is coated with a resin upon drawing, and is finally taken up by a take-up bobbin. At this time, the optical fiber is pulled while being held from its sides by a capstan. If the optical fiber is held by the capstan before the applied resin does not cure fully , then the applied resin may collapse and deform, thereby deteriorating transmission characteristics.
In recent years, the demand for optical fibers having a thicker resin coating has been increasing from the viewpoint of transmission characteristics, such as those for dispersion-shifted optical fibers for large-capacity transmissions, whereby the efficiency in the making thereof has been desired to improve. When increasing the drawing speed in order to enhance the productivity, the problem of collapsing has become evident. This problem occurs due to the fact that, as the resin coating is thicker, it becomes difficult for the applied coating to cool, and the curing reaction heat of the resin coating increases, whereby the applied coating becomes less likely to cool. Detrimental effects such as increases in dimensions of apparatus become greater if the pass line length is elongated more than necessary in order to fully cool the resin coating, whereas the efficiency in manufacture will decrease if the drawing speed is lowered for curing of the coated resin.
For satisfying both the quality of a made optical fiber and improvement of efficiency in manufacture at the same time at a high level, the inventors have carried out diligent studies in order to define an optimal pass line length according to a drawing condition for the optical fiber, and have found that the optimal pass line length corresponding to the drawing condition can be defined. The present invention is attained on the basis of the above-mentioned finding, and it is an object of the present invention to provide a method of drawing an optical fiber by which the efficiency in manufacture can be improved without deforming the resin coating.
The object of the present invention is to provide a method of drawing an optical fiber, in which an optical fiber with an outside diameter of 300 to 600 xcexcm after a resin coating is formed is drawn at a drawing speed of at least 50 m/min, the method comprising the step of drawing the optical fiber while forming the resin coating in a state where the pass line length zp from the most downstream outlet of a curing furnace for curing the resin coating to the most upstream entrance part of a capstan for pulling the optical fiber toward the downstream side satisfies the following expression (1):                               z          p                ≥                              -                                                                                ρ                    ⁢                                          xe2x80x83                                        ⁢                                          C                      p                                                        _                                ·                                  d                  2                                            4                                ·                                                    d                2                2                            -                              d                1                2                                      A                    ·                      V            f                    ·                      ln            ⁡                          [                                                                    (                                                                  T                        g                                            -                      10                                        )                                    -                                      T                    o                                                                                        T                    s                                    -                                      T                    o                                                              ]                                                          (        1        )            
where
Tg is the glass transition temperature of the coating resin [xc2x0 C.];
TS is the fiber temperature at z=0 [xc2x0 C.] [=TF(0)];
TO is the room temperature [xc2x0 C.];
{overscore (xcfx81Cp)} is the average heat capacity of glass and resin [J/(cm3xc2x7xc2x0 C.)];
d1 is the radius before coating [cm];
d2 is the radius after coating [cm];
A is a given constant within the range of 1.5xc3x9710xe2x88x926 to 4.5xc3x9710xe2x88x926 [J/(secxc2x7xc2x0 C.)]; and
Vf is the drawing speed [cm/sec].
The above-mentioned method of drawing an optical fiber may be carried out such that, when starting the drawing of an optical fiber, forming the resin coating, the pass line length zp is made shorter regardless of the above-mentioned expression (1) upon leading and is changed to a length satisfying the above-mentioned expression (1) upon production.